1. Field of the invention
The present invention relates to a compact zoom lens system of inner focus type consisting of three lens groups.
2. Description of the prior art
In the recent years where cameras are increasingly fully automated, lens systems permitting variations of focal lengths are adopted also as photographic lens systems for the so-called lens shutter cameras featuring portability. This trend has produced necessity to develop compact zoom lens systems.
It is possible to make compact the lens systems to be used in the lens shutter cameras by shortening back focal lengths since the lens systems for the lens shutter cameras are, unlike the lens systems for single lens reflex cameras, free from the restriction that definite back focal lengths must be reserved.
On the other hand, it is necessary to take into consideration, for designing compact lens systems for the lens shutter cameras, not only shortening of total lengths of the lens systems and minimization of diameters but also the relationship with the shutter mechanisms serving also as stops arranged in the lens barrels and the driving systems including the focusing mechanisms for automatic focusing which are arranged in the lens shutter cameras. Speaking concretely, the conventional lens system having a fixed focal length for use mainly at a wide angle, the lens system having switchable focal lengths and the zoom lens system having a low vari-focal ratio pose no problem since these lens systems can be made compact even when focusing is performed by shifting the lens system as a whole or the first lens group thereof. However, in case of the zoom lens system having a high vari-focal ratio, it comprises multiple lens groups and produces the defect described below when the first lens group is shifted for focusing:
The first lens group generally has a large diameter and must have refractive power weaker that those of the lens groups arranged thereafter for stabilizing the optical performance thereof, i.e., for minimizing variation of aberrations caused due to focusing. A first lens group having weaker refractive power inevitably increases shift distance for focusing even on an object located at the same distance. Accordingly, the shortest object distance allowed by the lens system is prolonged and the load imposed on the focusing motor arranged in the automatic focusing mechanism is increased, thereby increasing power consumption. Further, since the first lens group is located on the extremely object side in the lens barrel, it is difficult to control the first lens group directly from the side of the camera body, thereby making it necessary to use a driving motor for focusing independently of a shutter driving motor, and producing serious defects from the viewpoints of compactness and light weight of cameras as well as acceleration of focusing speed.